1. Field of the Invention
The present invention relates to a new and improved method and apparatus for altering the firing pattern of an existing furnace in order to produce lower levels of NO.sub.X to meet with the standards of the Clean Air Act. More particularly, the present invention relates to the alteration of the firing pattern of a cell-fired boiler or furnace without requiring new pressure part wall sections and without requiring the installation of an overfire air system with plenums separate and apart from the existing windboxes as is often required in retrofitting of furnaces to meet the new high standards of the Clean Air Act.
2. Background of the Prior Art
U.S. Pat. No. 4,245,980 discloses a burner for reduced NO.sub.X emission and control of flame spread and length wherein a burner tube is provided upstream of a first combustion zone for supplying a less than stoichiometric amount of combustion air in a tangential swirling pattern. The burner has a second plenum in coaxial relation with the first combustion zone so that tertiary air supplied tangentially to this plenum will flow in a helical swirling motion along the outside edge of the first combustion chamber around the downstream end.
U.S. Pat. No. 4,347,052 discloses a low NO.sub.X burner for firing liquid and gaseous fuels with a first air or oxidant plenum supplying primary air plus secondary air or oxidant to the liquid and/or gas burners and with the primary and secondary air providing a less than stoichiometic flow rate to provide a reducing atmosphere and preclude the formation of NO.sub.X.
U.S. Pat. No. 4,907,962 discloses a low NO.sub.X pulverized coal burner including a flow nozzle for injecting pulverized coal and primary air and a secondary air nozzle around and coaxial therewith along with a tertiary air nozzle arranged externally of the secondary air so that secondary and tertiary air swirl in a flow around the primary coal/air stream to delay mixing therewith.
U.S. Pat. No. 5,067,419 discloses a low NO.sub.X burner including a flame holding plate and an unburned gas burner provided in an exhaust gas duct to noticeably reduce the concentration of NO.sub.X discharged to the outside of the system.